In recent years, a secondary battery has been used for various products. A secondary battery includes a battery element in which a positive electrode, a separator, and a negative electrode are stacked. It is important for the battery element that the positive electrode and the negative electrode are stacked without positional deviation via the separator. This is because such positional deviation could deteriorate the performance of the battery or shorten the life of the battery.
Therefore, in order to prevent positional deviation between the positive electrode and the negative electrode, a technique to stack the positive electrode and the negative electrode rapidly and accurately by providing the positive electrode in a separator formed in a bag shape and the stacking a bag-shaped separator and the negative electrode has been suggested (refer to Patent Literature 1). According to this technique, the separator and the negative electrode are formed to have approximately same size and stacking the separator and the negative electrode by aligning the external sides enables accurate positioning of the negative electrode and the positive electrode inside the separator.